This study focuses on English loanwords in Thai, particularly the treatment of consonants in different environments, namely onset/coda simplification, laryngeal features, medial consonants, and liquid alternation, within the framework of Optimality Theory (OT: Prince and Smolensky 1993/2004). The major objectives are: (1) to examine the way English loanwords are adapted to a new environment, (2) to investigate how conflict between faithfulness and markedness constraints is resolved and in what ways through OT grammars, and (3) finally to be a contribution to the literature of loan phonology in OT since there has not been much literature on English loanwords in Thai within the recent theoretical framework of Optimality Theory. The data are drawn from an English-Thai dictionary (Sethaputa 1995), an on-line English-Thai dictionary, an English loanword dictionary (Komutthamwiboon 2003), and earlier studies of English loans in Thai by Udomwong (1981), Nacaskul (1989), Raksaphet (2000), and Kenstowicz and Atiwong (2004).